Sheathed or shielded resistance heaters have widely been used in many fields as heating parts because of their very excellent performance, quality and convenience. The commercial range of the heaters has now increased including not only domestic electric articles, but also specific applications such as in various industries, space developments and atomic power services. Among various classes of the sheathed electric heaters, sheathed heaters for high temperature purposes will more and more increase in applications.
Upon reviewing the performance and quality of sheathed resistance heaters in a world-wise sense, it will be found that they have defects in that the insulation resistance in a working condition (hereinafter referred to as insulation resistance under self-heating conditions) lowers as a function of time, coupled with another disadvantage in that it takes only a short time before breakage of the heating wire.